Context: Nitrogen-efficient fertilization on marginal sandy lands is crucial for enhancing agricultural productivity in degraded soils while promoting global food and oil security. However, the relationships between nitrogen (N) regimes, root-soil interactions, and tuber quality remain poorly understood. Objective: This study aims to elucidate how N fertilization modulates root adaptive strategies, soil nutrient availability, and extracellular enzyme activity, thereby influencing tuber yield and quality in tiger nut (Cyperus esculentus L.) grown on sandy farmland. Methods: The experiment was conducted in sandy farmland with five nitrogen (N) application treatments: no nitrogen (N0), 100 (N100), 200 (N200), 300 (N300) and 400 (N400) kg N ha(-1). We systematically investigated: root functional traits, soil properties (total nitrogen, inorganic nitrogen, and organic matter), extracellular enzyme (beta-glucosidase (beta G), beta-D-cellobiosidase (CBH), beta-1,4-N-acetylglucosaminidase (NAG), beta-1,4-xylosidase (XYL), L-leucine aminopeptidase (LAP)) and tuber parameters (yield, crude fat, protein and starch). Partial least squares structural equation modeling (PLS-SEM) was employed to analyze the relationships between soil properties and plant performance. Results: Our results revealed divergent root adaptation strategies across nitrogen (N) gradients. Under N0, tiger nut plants prioritized resource allocation toward thinner, elongated roots, significantly increasing specific root length (24.24 % - 372.63 %) and area (35.73 % - 385.22 %). Conversely, nitrogen-sufficient regimes (N300-N400) promoted denser root architectures, with root area and length densities increasing by 18.27 % - 57.42 %. This morphological shift coincided with significant soil enrichment; N300-N400 levels elevated soil inorganic nitrogen, total nitrogen, and organic matter, while stimulating beta G and NAG activities. However, soil pH and CBH declined, and XYL activity peaked specifically at N300. Consequently, tuber yield reached a maximum at N300 before plateauing at N400. High nitrogen levels further improved quality by boosting crude protein (35.41 % - 42.47 %) and oil content (10.37 %-11.56 %), despite a concurrent reduction in starch content. Conclusions: This study demonstrates the synergy between root morphological plasticity and soil biochemical health in boosting tiger nut productivity. Strategic nitrogen management stimulates adaptive root architecture and enhances soil enzymatic activity and nutrient availability in nutrient-poor environments. A critical threshold of 300 kg N ha(-1) was identified, providing a framework to transform marginal sandy soils into productive, high-quality systems. These findings offer a sustainable pathway for cultivating climate-resilient crops, strengthening food security, and restoring degraded farmlands.